Recently it has been found elsewhere and confirmed in part of our study group that the leanest persons have substantially higher mortality rates than those of average build. this is surprising in view of previous evidence as to the health benefits of leanness in humans and the added lifespan noted among experimental animals on restricted caloric intakes. In order to explain this finding we propose a multifaceted epidemiologic investigation of some 45,000 persons who received three or more multiphasic health checkups between 1964 and 1973 and who will be followed up for death through 1979, an average of 9.5 years per person. Height, weight, weight change and abundant other health-related data will be available from thse computer-stored checkups and from manual review of medical records of decedents. We shall determine whether the thinness-mortality association is an artifact representing weight loss due to illness already present when weight was measured, by analyzing weights earlier in life, weight trends, and health status at the time of weighing. We shall examine the causes of death among thin people and among an averge-build comparison group to determine which diseases account for the difference in rates; we would then determine whether the thin have a higher incidence of, or a poorer survival with these diseases. We shall also better characterize the type of thinness associated with mortality, both as to duration of thinness and body build. Finally, we will analyze a number of confounding factors to determine whether they account for the relation of thinness to mortality from all and specific causes. This study will indicate those aspects of thinness, if any, that are harful to health and will provide guidelines to the medical profession and to our weight-conscious society concerning weight for a given height that is optimal for longevity.